There has been much effort in the coatings industry to develop application specific coatings, ie., coatings that exhibit chemical and physical properties necessary for dependable coating performance under selected usage conditions. The property of impact resistance in a coating composition requires a dynamic balance between hardness and flexibility that allows the coating to endure points of impact, typically of a force sufficient to deform not only the coating but as well the underlying surface, without the coating breaking, chipping or cracking, or losing adhesion to the underlying surface. Exemplary of applications requiring impact tolerance is in coatings used in certain sporting equipment, for example, on golf balls and bowling pins, as well as coatings targeted for application to vehicle bumpers and other surfaces subjected to impact deformation in normal use.
The present invention provides a coating formulation capable of providing impact resistant/tolerant coatings exhibiting good adhesion and resistance to being marred, chipped, cracked or deformed at points of high impact. In one embodiment of this invention the coating compositions are formulated for application as a clear top coat for golf balls. Such coating compositions in accordance with this invention include combinations of mono- and polyethylenically unsaturated resin solids, an effective amount of a radical or cationic polymerization initiator to polymerize the resin composition and an effective amount of an optical brightener. Optionally the coating composition in accordance with this invention can be pigmented.
The coating compositions in accordance with this invention are adapted for application by any of the art-recognized coating application techniques, including spray application to provide coated surfaces with high impact tolerance. The present coating compositions can be formulated as solvent-free compositions using reactive diluents to provide viscosity control or they can comprise a solvent, typically a mixture of ketone and ester solvents commonly used for solvent-based coatings, and about 30% to about 100% by weight of ethylenically unsaturated resin solids. Preferred ethylenic unsaturated groups are acrylate, methacrylate and vinyl ethers. The radical initiated resin solids are selected to include a mixture of both monoethylenically unsaturated monomers and polyethylenically unsaturated monomers, most typically a multiplicity of polyethylenically unsaturated resins, each selected for its contribution to the aggregate properties of the polymerized coating product. The cationic initiated resin solids can include polyols, vinyl ethers and cycloaliphatic epoxies. In one preferred embodiment, the components are also selected for their optical purity and non-yellowing characteristics, particularly for use in non-pigmented compositions.
A polymerization initiator is included in the composition in an amount effective to polymerize the resin composition. The initiator can be a cationic polymerization initiator or a radical initiator depending on the resin reactants and is typically activated by actinic radiation, for example, UV radiation. Alternatively, for some applications, the initiator can be a compound that initiates polymerization at ambient or elevated temperatures. Again, such components are selected so that they will have minimum impact on coating clarity and so that they will not engender or augment radiation-induced yellowing of the polymerized coating composition. In addition, coating compositions of this invention formulated, for example, as a top coat for golf balls can include any one of a wide variety of art-recognized optical brighteners.
The cured coatings prepared using the preferred coating compositions of the present invention exhibit superior high impact and abrasion resistance, excellent adhesion, and good color retention upon prolonged exposure to UV radiation.